my 2 cents
This is awesome scary. In the article I cited earlier it talked about a miner withholding their chain while waiting for the next block to be found and then publishing it - e.g. I have 51%, I find the next block before anyone else, but I don't publish, rather I start working on next block for a say a minute and then publish the block, as long as no one else found the block and published it, I now have a minute head start on the next block, rinse and repeat. But can they really hold it for a whole month?
You have described what is called the Selfish Miner attack, by Eyal and Sirer from their paper
Majority is not Enough: Bitcoin Mining is Vulnerable. You find the block, but hold on to it for a bit while you work on the next block to improve your chances at getting the next block since you have more time to mine it. Imagine that on a 51% scale. You will increase your earnings, your pool performs better, you attract more to your pool with the promise of riches.....rinse and repeat. You could potentially become the only minter of new bitcoins. Why even double spend when you can get the monopoly on minting every new coin? There would technically be no flaw in the protocol, you are just taking a chance as to when to publish the block.
"Rational Bitcoin Attackers want to see Bitcoin succeed": this is dodgy. Insofar as Bitcoin becomes a significant part of western infrastructure, it will become a target for those who want to harm the west. Think of the attack that brought down the World Trade Centre. Claiming such people are irrational isn't helpful.
The Goldfinger attack, described in the paper
The Economics of Bitcoin Mining, of Bitcoin in the Presense of Adversaries by Kroll, Davey and Felten, comes to mind here. Goldfinger uses his vast resources to destabilise Bitcoin which causes it to crash, be it in owning say the monopoly on power companies and increasing power prices, or buying available BTC and selling cheap, or having political clout and declaring it illegal, or having a trusted reputation and saying Bitcoin is "bad." An interesting concept with the Goldfinger attack is that it is possible to do with just the threat of using your resources. Reflexivity and/or self fulfilling prophecy come to mind.